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Dr. Philipp Ehses

Adresse: Spemannstr. 41
72076 Tübingen
Raum Nummer: 3.A.02
Tel.: 07071 601 937
Fax: 07071 601 702
E-Mail: Philipp.Ehses

 

Bild von Ehses, Philipp, Dr.

Philipp Ehses

Position: Gastwissenschaftler  Abteilung: Scheffler

Distortion-free high-resolution fMRI at 9.4 T


Introduction

Signal-to-noise-ratio (SNR) in MRI is expected to grow linearly with increasing field, while a BOLD increase of more than linear is expected [1], allowing a significant higher resolution in BOLD fMRI at ultra-high fields [2]. On the flip side, increased B0 field inhomogeneities aggravate distortion artifacts in EPI BOLD imaging at higher field strengths. Additionally, stronger T2* relaxation during the EPI read-out leads to increased blurring effects, partially offsetting the higher nominal resolution. Both of these problems can be alleviated by shortening the EPI read-out using parallel imaging and/or segmentation. In the extreme case, only a single echo is acquired in every TR, transforming the EPI into a conventional gradient-echo (GRE) sequence. The aim of this work is to develop an effective single-echo GRE sequence for BOLD fMRI at ultra-high magnetic field. To this end, echo-shifting (ES) [3] was combined with an interleaved slice order, in order to optimize echo time, scan time, and SNR for multi-slice BOLD fMRI.

 

Methods

All experiments were performed at 9.4 T on a healthy volunteer with informed consent and IRB approval. A custom-built head coil was used for signal transmission/reception (8 transmit / 18 receive channels). An echo-shifted GRE with interleaved slices was developed, using an acquisition strategy that was recently proposed for an EPI sequence [4]. Parallel imaging (R=2 using GRAPPA [5] with ACS pre-scan) and partial Fourier (6/8) were used to accelerate image acquisition. Other sequence parameters were as follows: FA = 22°, effective TR/TE = 80/20 ms with ten interleaved slices and two echo-shifts, slice thickness = 1 mm (+25% slice distance), 1 mm in-plane resolution, TA per repetition = 4.32s, 70 repetitions, total TA = 5min. The paradigm consisted of finger tapping with the right hand, alternating between 20 s of rest and 20 s of tapping. For analysis, the data were processed with FSL FEAT [6,7] after brain extraction, using a standard hemodynamic response function and temporal filtering. Data processing was repeated with and without spatial smoothing, using a Gaussian filter kernel (FWHM=1mm). In addition, a high-resolution GRE image served as a reference (FA = 25°, TR/TE = 500/14ms, resolution = 0.23x0.23 mm2, slice thickness = 1mm, 16 slices). Activation maps were registered to this reference using a six-parameter rigid body model.

 

Initial Results

Figure 1 shows the time series without spatial smoothing for the voxel with maximum z-score. A strong BOLD activation (±40%) is observed, explained by the fact that the corresponding voxel lies inside a vein. Typical activations in the motor cortex were approximately ±4%. Figure 2ab shows an overlay of the smoothed activation map over a) the averaged raw image and b) registered to the reference image. In Figure 2c-e, the left motor cortex is superimposed with activation maps with and without spatial smoothing. The position of veins close to the motor cortex strongly correlates with high BOLD activation.

 

Discussion/Conclusion

The results show that the proposed method is well suited for high-resolution fMRI at ultra-high magnetic field. Advantages compared to EPI are that it results in less T2* blurred and virtually distortion-free images. This comes at the cost of longer scan times (4.32 s for 10 slices vs. 2s for a typical >=20 slices EPI). Further optimizations and higher acceleration factors may help to close this gap, although we believe that EPI will remain more time-efficient. Another possible advantage for auditory fMRI applications is that the proposed sequence creates very monotonous, mostly low frequency noise. Interestingly, and apart from its advantages compared to EPI, the proposed method also becomes more feasible with higher field: Due to shorter T2*, less echo-shifts (i.e. less gradient switching and diffusion related signal loss) are required to optimize the echo time for BOLD.

 

References

  1. Gati JS, Menon RS, Ugurbil K, Rutt BK. Experimental determination of the BOLD field strength dependence in vessels and tissue. Magn Reson Med 1997;38:296-302.
  2. Speck O, Stadler J, Zaitsev M. High resolution single-shot EPI at 7T. MAGMA 2008;21:73-86.
  3. Moonen CTW, Liu G, Van Gelderen P, Sobering G. A fast gradient-recalled MRI technique with increased sensitivity to dynamic susceptibility effects. Magn Reson Med. 1992;26:184–189.
  4. Gibson A, Peters AM, Bowtell R. Echo-shifted multislice EPI for high-speed fMRI. Magn Reson Imaging. 2006;24:433-42.
  5. Griswold MA, Jakob PM, Heidemann RM, Nittka M, Jellus V, Wang J, Kiefer B, Haase A. Generalized autocalibrating partially parallel acquisitions (GRAPPA). Magn Reson Med 2002;47:1202-10.
  6. Smith SM, Jenkinson M, Woolrich MW, Beckmann CF, Behrens TEJ, Johansen-Berg H, Bannister PR, De Luca M, Drobnjak I, Flitney DE, Niazy RK, Saunders J, Vickers J, Zhang Y, De Stefano N, Brady JM, Matthewsa PM. Advances in functional and structural MR image analysis and implementation as FSL. NeuroImage 2004;23:208–19.
  7. Woolrich MW, Jbabdi S, Patenaude B, Chappell M, Makni S,  Behrens T, Beckmann C, Jenkinson M, Smith SM. Bayesian analysis of neuroimaging data in FSL. NeuroImage 2009;45:173-86.

 

 

 

 

 

Figure 1: Time series plot of the voxel with maximum z-score (=11.4) (without spatial smoothing). The very strong BOLD response can be explained by the fact that the voxel lies directly inside a vein.

 

 

 

Figure 2: a) Overlay of BOLD activation (smoothed with FWHM=1mm) over averaged raw image. b) Activation from a) registered to reference. c-e) Zoom in on left motor cortex (different slice than in ab), superimposed with d) registered activation map (FWHM=0) and e) registered and smoothed map (FWHM=1mm). Veins close to the motor cortex are indicated by white arrows.

 

since 08/2011

Scientist at the Max-Planck-Institute for Biological Cybernetics & at the Department for Neuroimaging, University of Tübingen, Germany.

2006 - 2011

PhD student at the Department for Experimental Physics 5, Julius-Maximilians-Universität Würzburg, Germany. Thesis supervisor: Prof. Dr. Peter Jakob. Thesis title: "New Strategies for Fast Parameter Mapping in Magnetic Resonance Imaging". (defense pending)

01 - 07/2009

Research Scholar at the "Case Center for Imaging Research", Case Western Reserve University, Cleveland, Ohio, USA.

2000 – 2005

Diploma in physics. Julius-Maximilians-Universität Würzburg, Germany.

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Artikel (14):

Wright KL , Lee GR , Ehses P Person, Griswold MA , Gulani V und Seiberlich N (Oktober-2014) Three-dimensional through-time radial GRAPPA for renal MR angiography Journal of Magnetic Resonance Imaging 40(4) 864–874.
Benkert T , Ehses P Person, Blaimer M , Jakob PM und Breuer FA (Januar-2014) Dynamically phase-cycled radial balanced SSFP imaging for efficient banding removal Magnetic Resonance in Medicine Epub ahead.
Choli M , Blaimer M , Breuer FA , Ehses P Person, Speck O , Bartsch AJ und Jakob PM (August-2013) Combined acquisition technique (CAT) for high-field neuroimaging with reduced RF power Magnetic Resonance Materials in Physics, Biology and Medicine 26(4) 411-418.
Pohmann R Person, Shajan G Person, Hoffmann J Person, Budde J Person, Hagberg G Person, Bieri O , Bisdas S , Ernemann U , Weigel M , Ehses P Person, Hennig J , Chadzynski G und Scheffler K Person (April-2013) Imaging and Spectroscopy at 9.4 Tesla: First Results on Patients and Volunteers MAGNETOM Flash 2013(2) 58-67.
Ehses P Person, Seiberlich N , Ma D , Breuer FA , Jakob PM , Griswold MA und Gulani V (Januar-2013) IR TrueFISP with a golden-ratio-based radial readout: Fast quantification of T1, T2, and proton density Magnetic Resonance in Medicine 69(1) 71–81.
Seiberlich N , Lee G , Ehses P Person, Duerk JL , Gilkeson R und Griswold MA (Dezember-2011) Improved temporal resolution in cardiac imaging using through-time spiral GRAPPA Magnetic Resonance in Medicine 66(6) 1682–1688.
Seiberlich N , Ehses P Person, Duerk J , Gilkeson R und Griswold M (Februar-2011) Improved radial GRAPPA calibration for real-time free-breathing cardiac imaging Magnetic Resonance in Medicine 65(2) 492-505.
Lopez MA , Ehses P Person, Breuer FA , Ponce IP , Gareis D und Jakob PM (Oktober-2010) A four-channel hole-slotted phased array at 7 Tesla Concepts in Magnetic Resonance B: Magnetic Resonance Engineering 37B(4) 226-236.
Nordbeck P , Weiss I , Ehses P Person, Ritter O , Warmuth M , Fidler F , Herold V , Jakob PM , Ladd ME , Quick HH und Bauer WR (März-2009) Measuring RF-induced currents inside implants: Impact of device configuration on MRI safety of cardiac pacemaker leads Magnetic Resonance in Medicine 61(3) 570-578.
Seiberlich N , Breuer FA , Ehses P Person, Moriguchi H , Blaimer M , Jakob PM und Griswold MA (März-2009) Using the GRAPPA operator and the generalized sampling theorem to reconstruct undersampled non-Cartesian data Magnetic Resonance in Medicine 61(3) 705-715.
Ehses P Person, Fidler F , Nordbeck P , Pracht ED , Warmuth M , Jakob PM und Bauer WR (August-2008) MRI thermometry: Fast mapping of RF-induced heating along conductive wires Magnetic Resonance in Medicine 60(2) 457-461.
Ceymann A , Horstmann M , Ehses P Person, Schweimer K , Paschke A-K , Steinert M und Faber C (August-2008) Solution structure of the Legionella pneumophila Mip-rapamycin complex BMC Structural Biology 8(17) 1-12.
Nordbeck P , Fidler F , Weiss I , Warmuth M , Friedrich MT , Ehses P Person, Geistert W , Ritter O , Jakob PM , Ladd ME , Quick HH und Bauer WR (August-2008) Spatial distribution of RF-induced E-fields and implant heating in MRI Magnetic Resonance in Medicine 60(2) 312-319.
Horstmann M , Ehses P Person, Schweimer K , Steinert M , Kamphausen T , Fischer G , Hacker J , Rösch P und Faber C (Oktober-2006) Domain Motions of the Mip Protein from Legionella pneumophila Biochemistry 45(40) 12303-12311.

Beiträge zu Tagungsbänden (1):

Balla DZ Person, Ehses P Person, Pohmann R Person, Budde J Person, Mirkes C Person, Shajan G Person, Bowtell R und Scheffler K Person (Juli-2013) Functional QSM at 9.4T with single echo gradient-echo and EPI acquisition 2nd International Workshop on MRI Phase Contrast & Quantitative Susceptibility Mapping (QSM 2013), 1-4.
pdf

Poster (8):

Ehses P Person, Shajan G Person und Scheffler K Person (Mai-15-2014): 3D Radial GRE-EPI with Up to 8-Fold Acceleration for Functional Imaging at 9.4T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.
Chadzynski GL Person, Henning A Person, Ehses P Person, Hoffmann J Person, Shajan G Person und Scheffler K Person (Mai-15-2014): High-Resolution Free Induction Decay Proton MRSI in the Human Brain at 9.4 T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.
Bause J Person, Ehses P Person, Shajan G Person, Scheffler K Person und Pohmann R Person (Mai-12-2014): Functional ASL at 9.4 T: a Comparison Between Balanced SSFP and GRE-EPI Readout, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.
Balla DZ Person, Ehses P Person, Pohmann R Person, Budde J Person, Mirkes C Person, Shajan G Person, Bowtell R und Scheffler K Person (Juli-2013): Functional QSM at 9.4T with single echo gradient-echo and EPI acquisition, 2nd International Workshop on MRI Phase Contrast & Quantitative Susceptibility Mapping (QSM 2013), Ithaca, NY, USA.
pdf
Ott M , Blaimer M , Ehses P Person, Jakob PM und Breuer F (Mai-7-2012): hase sensitive PC-bSSFP: simultaneous quantification of T1, T2 and spin density M0, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia.
Neumann D , Seiberlich N , Breuer FA , Lee G , Ehses P Person, Duerk JL , Jakob PM und Griswold MA (Mai-7-2012): Moët: Multiple oscillating ëfficient trajectories, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia.
Ehses P Person, Budde J Person, Shajan G Person und Scheffler K Person (Mai-2012): Distortion-free high-resolution fMRI at 9.4 T, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia.
Breuer F , Ehses P Person, Seiberlich N , Blaimer M , Jakob P und Griswold M (Mai-2012): High quality Real-Time Cardiac MRI using Self-Calibrating Radial GRAPPA with Sparsification, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia.

Vorträge (2):

Ehses P Person, Budde J Person, Shajan G Person und Scheffler K Person (April-24-2013) Abstract Talk: T2-weighted BOLD fMRI at 9.4 T using a S2-SSFP-EPI sequence, 21st Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2013), Salt Lake City, UT, USA 1090.
pdf
Ott M , Ehses P Person, Blaimer M , Benkert T , Jakob P und Breuer F (Oktober-2012) Abstract Talk: Towards a full 3D MR parameter quantification by means of (demodulated) off-resonance insensitive balanced SSFP, 29th Annual Scientific Meeting ESMRMB 2012, Lisboa, Portugal, Magnetic Resonance Materials in Physics, Biology and Medicine, 25(Supplement 1) 63.

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Last updated: Dienstag, 23.09.2014